2 Answers
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It's going to be marked as 'E85' on the pump. It's not that common - I've probably seen one gas station in Vegas that sold it, plus up here in Northern NV I know of one or two stations that sell it - and you'll notice that it's cheaper than regular gas usually. Plus the pumps usually carry large warning stickers, too.

It's a blend of up to 85% Ethanol with at least 15% gasoline. The exact mixture tends to vary a bit (IIRC there's usually more gasoline in it during winter as that improves starting). The main problem with it is that the stochiometric ration for E85 is much lower compared to pure gasoline (around 9:1 air/fuel instead of 14.7:1) so an engine that isn't set up to run E85 will run massively lean. That's the main reason your mower suggests it doesn't want to be run on E85.

E85 in the most part tends to be common in the Midwest mainly because that's where corn is grown the most, so it's much less costly to ship to stations/refineries. E85 is a fairly new fuel made from 85% ethanol (corn) and 15% crude oil. GM was one of the first companies to embrace it on the consumer side through their FlexFuel vehicles, however it's been common for trucks and buses which are fairly easy to modify to use biodisel.

E85 doesn't pack as much power as true gas which is why it's only usable in modified engines, and also it provides a bit less milage, although per gallon it's cheaper at the pump than regular.

Overall, if a gas station is carrying E85, it will likely have it's own pump (similar to how Diesel is separate from the regular/plus/premium tanks) and the price will likely look ridiculously low -- i.e. a gas station near me had the E85 price where Regular normally would be, so at first glance you thought the Regular gas was $3.00 and the $3.85 price was for premium, but it really was regular)

In terms of popularity, in New York I've seen it at some stations along the LIE (one of the busiest highways on Long Island, especially for trucks) but I have yet to see it elsewhere.

I just saw my first ever E85 pump the other day here in Ohio. It was bright yellow, with "E85" all over it. Very easy to identify. :-)
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Brian KnoblauchAug 8 '11 at 12:07

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When I first saw my E85 pump, it was at a Gulf station right off the LIE exit I take when going to my client's office, and it actually had mini corn on the cob pictures on the left and right sides.
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theonlylosAug 8 '11 at 17:36